1. Field of the Invention
The present invention relates to a method of stabilising an output delivered by a transducer in the case where the output is proportional to an approximately integral power of an input provided to the transducer, and to apparatus for carrying out the method.
The present invention relates especially to a method of stabilising the light output from a transducer such as a cathode-ray tube used to provide a picture display either for direct viewing or photographic exposures.
2. Description Relative to the Prior Art
Methods currently employed to control the light output from a cathode-ray tube use measurements of the cathode current or light output corresponding to two input signal levels representative of two brightness levels. It is usual to choose white and picture-black for these brightness levels. In a display system using conventional black-level clamping to control light output from a cathode-ray tube, the adjustment of black level affects the white-display conditions and the correct control settings have therefore to be approached by means of successive adjustments of the gain and black-level controls. The light output from white areas is made less dependent upon adjustments in picture black-level in the white-level clamping technique described in B.B.C. Research Department report PH-22 1968/38, "Control of the Picture Display in Colour Film Recording Equipment."
In using any of these methods, there are two variables--video gain and the bias potential of the cathode-ray tube--that are available to carry out the necessary adjustment of brightness levels and maintain the operating stability of the cathode-ray tube so as to correct its light output for long term drift. In current practice such as that described in the B.B.C. report the gain and bias corrections interact and consequently a number of operations may be necessary to reach a steady state.